Development of Post-Accident Monitoring System for Severe Accidents

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1 Development of Post-Accident Monitoring System for Severe Accidents Chang-Hwoi Kim a, Sup Hur b, Kwang-Sub Son c, and Tong-Il Jang d a b c d Abstract: To cope with a severe accident such as Fukushima Nuclear power plants, fully independent monitoring and control system separated (isolated) from the conventional instrumentation and control system is needed. Also, a remote control room which is movable and usable at a distant location is needed for safe plant control and monitoring in emergency. In this paper, we will suggest a new concept for remote mobile control room and hardened I&C equipment to cope with a severe accident in nuclear power plants. Keywords: Severe Accident, Hardened I&C equipment, Mobile Control Room, Fukushima Accident, Emergency Facility. 1. INTRODUCTION The Fukushima Dai-ichi and Dai-ni NPP were hit by the Off the Pacific Coast of Tohoku Earthquake ( the Earthquake ) and accompanying tsunami waves ( the Tsunami ). The Earthquake was of Magnitude 9.0 and the Tsunami waves height at the Fukushima Dai-ichi NPP exceeded 15 meters. Six nuclear power units stood at the Fukushima Dai-ichi NPP: Units 1 to 3 were in operation, and Units 4 to 6 were in maintenance modes at the time of the Earthquake. Units 1 to 3 appeared to have automatically scrammed at the Earthquake, but external power supplies and almost all in-house AC power supplies were lost due to the Earthquake and the Tsunami. Reactors and spent fuel pools at the Fukushima Dai-ichi NPP lost their cooling capabilities. Explosions occurred on Units 1, 3 and 4, which were caused presumably by the hydrogen released from the possible core damage and filled in the reactor buildings. The reactor core of Unit 2 also seems to have been damaged, although the investigation is still incomplete. A large amount of radioactive materials were released and spread from the Fukushima Dai-ichi NPP. The zone up to 20km from the site was designated as the Access Restricted Areas and no entry is allowed unless authorized. Some areas outside 20km from the site were also designated as the Deliberate Evacuation Areas. As many as more than 110,000 people have evacuated. Many people are still forced to live in evacuation, and radiation contaminations have caused serious impacts in extended areas. The fundamental problem at the Fukushima Dai-ichi plant after the accident is lack of information about the true situation of the units. Especially, the power supply was failed and an accident-proof back-up battery system was not prepared. And the monitoring system for the emergency case was not designed for these types of accidents. To cope with a severe accident including Fukushima cases, fully independent monitoring system separated (isolated) from the conventional instrumentation and control system is needed. Also, a remote control room which is movable and usable at a distant location is needed for safe plant control and monitoring in emergency.

2 Korea Atomic Energy Research Institute (KAERI) has started a new project to cope with these problems. Hardened monitoring equipment that consumes low power and survives extreme environment such as high temperature, high radiation, and strong earthquake is developing. And, a remote mobile control room to observe plant situation in off-site area is developing. The hardened monitoring equipment and remote mobile control room is conceptually connected through wireless communication and satellite communications. 2. SYSTEM DESCRIPTION The configuration of the post-accident monitoring system and conventional I&C system is as figure 1. The post-accident monitoring system is consisted of hardened monitoring equipment, Remote mobile control room, and satellite (wireless) communication. The severe accident related sensor signals such as reactor temperature, reactor pressure, coolant flow, and radioactivity are distributed to hardened monitoring equipment via signal isolators. Figure 1: Configuration of the Post-Accident Monitoring System Main Control Room Hardened Monitoring Equipment I&C Equipment Isolator Actuators Sensors Remote Mobile Control Room Actuators If the MCR(Main Control Room) and/or conventional I&C equipment is failed caused by severe accident such as natural disasters, radiation leak, or station blackout, etc., the relevant parameters inside containment and spent fuel pool reactors during and after a severe accident can t be monitored like as the Fukushima Dai-ichi NPP. In this case, the hardened monitoring equipment acquires and processes severe accident related signals and then sends the signals to remote mobile control room using wireless (satellite) communication. The capabilities of the remote mobile control room can monitor multi-units severe accident and control actuators necessary for severe accident mitigation operation. To evaluate of the functional performance for the post-accident monitoring system, we installed a testbed. The test-bed is consisted of nuclear power plant simulator and I/O interface equipment. The figure 2 shows test environment.

3 Figure 2: Test Environment for the Post-Accident Monitoring System 3. REQUIREMENTS The top tier requirements for the post-accident monitoring system are as follows: The hardened monitoring equipment is fully independent monitoring system separated (isolated) from the conventional instrumentation and control system The hardened monitoring equipment uses independent power supply from own rechargeable battery for 100+ operation hours The hardened monitoring equipment is not protection system. But the environment condition test (temperature, seismic/vibration, EMI/EMC/EMP, water proof, etc.) will be performed. The hardened monitoring equipment is able to measure severe accident related parameters such as temperature (RTD/TC), pressure, water level, radiation, hydrogen concentration, and flow, etc. The hardened monitoring equipment transmits measured parameters to remote mobile control room using wireless communication. The remote mobile control room will be located in off-site area about 30km. The remote mobile control room has monitoring capability all nuclear power units in the site respond to the multi-units accidents (maximum 10 nuclear power units). Effective display for selected-accident parameters and control to properly the plant subsequently to severe accident. The remote mobile control room has ability of organic connection with Emergency Operating Facility (EOF). The remote mobile control room has data storage and recovery capability for accident analysis Secure and reliable data communication between hardened monitoring equipment and remote mobile control room is provided. The post-accident monitoring system shall be non-safety system. But seismic classification is seismic category 1. The table 1 shows severe accident related parameters for KSNP (Korea Standard Nuclear Power Plant). Table 1: Severe accident related parameters for KSNP Variables Purpose

4 1 Core Exit Temperature Evaluation of Core Cooling Function and Damage 2 Rx Vessel Water Level 3 SG Water Level Determine the Core Exposure. Monitoring of Coolant Injection. Evaluation of loss of 2ndary heat sink source. Information for determination to use alternate heat sink source. 4 RCS Pressure Monitoring of Coolant Boiling. 5 CV Pressure Verification of LOCA and CV Integrity. 6 CV Temperature Verification of LOCA and CV Integrity. 7 CV Hydrogen Concentration Determination of Core melt and CV Ventilation. 8 9 ICIS Thimble Tube Room Water Level CV Refueling water level in CV Circulation Sump Water Level Verification(Monitoring) of Cooling Source for Melted Core Verification of Emergency Core Cooling Source 10 Monitoring Post Assessment of Regional Radioactivity 11 High Range Area Radiation Monitor Assessment of Radioactivity in CV 12 Exhaust High Range Gas Monitor Evaluation of Radioactivity of Ventilation Gas 13 Main Steam Line Monitor Monitoring of SG Tube Rupture 15 NIS Determine Sub-criticality (of reactor core) 16 CCW flow Assessment of Functionality of Heat Sink Source The table 2 shows requirement and specification of the hardened monitoring equipment and conventional safety/non-safety I&C system. Table 2: Requirement and specification for the hardened monitoring equipment Conventional I&C System Environment Specification Non-safety System Safety System Radiation 10 Gy/40years 10 Gy/40years 2 kgy/100hr Temperature C 10 ~ 60 C 10 ~ 80 C Humidity 10-90% 10 ~95 % 100 % Seismic Not consider 10 G 15 G Harden monitoring equipment environment Specification EMI/EMC Power consumption (EMI/EMC) (EMI/EMC/ESD/Surge) (EMI/EMC/ESD/Surge) 250 W/Rack 150 W/Rack > 40 W/Rack Explosion- Not consider Not consider EXeIICT5 /100%

5 proof 4. CONCLUSION To develop hardened monitoring equipment, we are performing gamma irradiation test for electric device. A power regulators, OP AMPs, logic devices, communication devices, AD and DA converters, micro-processors and etc., were tested and successfully selected according to the irradiation specification. Using the selected device, we are developing pre-prototype for hardened monitoring equipment. The pre-prototype for remote mobile control room is developed using multi touch screen and table PC for monitoring multi-units accidents.